Temperature change will have minimum effect on signal integrity since the AM335x DDR IO buffers are temperature compensated. The PCB design can have significant effects on signal integrity, so you should follow the guidelines in the data sheet.
The AM335x device will operate with DDR3 under these conditions if the PCB is designed such that it does not cause signal integrity issues.
I also recommend you to perform a thermal analysis of your product to insure the silicon devices stay within their recommended operating range when exposed to a 85C ambient. Review the following thermal considerations.
http://processors.wiki.ti.com/index.php/AM335x_Thermal_Considerations
Regards,
Paul